I have read some, yes. He was a complicated person with complicated opinions and many decades to shift his worldviews. Early in life he was an open admirer of the Social Credit Party, the turn of last century Universal Basic Income party, so he was an open socialist for some years.

It's easy to chalk his early socialist inclinations as youthful vigor that maybe he regretted later in life, but that isn't the only way to read it. Despite what modern political in-fighting and team-formations may wish us to believe, socialist and libertarian are not mutually exclusive constructs (one is socioeconomic and the other sociopolitical, and there can be and is an intersection; UBI is very libertarian as an ideal/project). Up until the Red Scare and McCarthyism there were American socialists on both sides of the aisle. After the Red Scare there were still socialists on both sides of the aisle, but yes, most of them at that point were forced into the closet and out of mainstream discussion for many decades in the later part of the twentieth century.

I think you misreprepresent history. The people calling themselves "libertarian socialists" in the early part of the 20th century were not "on the other side of the aisle" exactly, they were almost all anti-capitalist. And if you're talking about the literal U.S. major political parties, there was not the "Republicans==conservative/right and Democrats==liberal/left" association then, what literal congressional "side of the aisle" did not have the left/right liberal/conservative association it does now, party politics were different. And "libertarian" didn't mean then what it does now in the U.S. either.

But yes, a variety of politics are possible, not just two, and Heinlein certainly had his own odd one, agreed.

In Starship Troopers, of course, everyone gets a kind of UBI I think, but only those who serve in the military have the political franchise. That was a fairly early work of his, and he definitely kept getting weirder from there.